1. Field of the Invention
The present invention relates to surface acoustic wave sensors, and more specifically, to surface acoustic wave sensors utilizing a change in the frequency characteristics of surface acoustic wave elements.
2. Description of the Related Art
To date, various surface acoustic wave sensors have been proposed which utilize a phenomenon in which the frequency characteristics of a surface acoustic wave element change with a change in the mass load on the surface of an excited portion of the surface acoustic wave element.
For example, WO 2005/003752 discloses a surface acoustic wave sensor 101 illustrated in FIGS. 10A to 10D. In the surface acoustic wave sensor 101, an interdigital transducer (IDT) electrode 103 defining a surface acoustic wave element and a reactive layer 104 are provided, as illustrated in FIGS. 10A and 10C, and the IDT electrode 103 is covered by the reactive layer 104.
Referring to FIG. 10A, when a liquid 105 is in contact with the reactive layer 104, frequency characteristics detected by the surface acoustic wave element including the IDT electrode 103 change. For example, referring to a wave diagram illustrated in FIG. 10B, frequency characteristics denoted by symbol A change to frequency characteristics denoted by symbol B.
Referring to FIG. 10C, when the liquid 105 in contact with the reactive layer 104 contains a material 106 which bonds with the reactive layer 104, the mass load on the IDT electrode 103 is increased as a result of the material 106 bonding with the reactive layer 104 as compared to when the liquid 105 does not contain the material 106. Thus, the frequency characteristics detected by the surface acoustic wave element including the IDT electrode 103 change from the frequency characteristics denoted by symbol A to frequency characteristics denoted by symbol C in the waveform diagram illustrated in FIG. 10D.
In this manner, the material 106 can be detected based on the fact that the frequency characteristics B and C are different in accordance with the existence/nonexistence of the material 106 which bonds with the reactive layer 104.
Japanese Unexamined Patent Application Publication No. 2007-57287 discloses a surface acoustic wave sensor illustrated in FIGS. 11A and 11B. FIG. 11A is a schematic plan view. FIG. 11B is a sectional view taken along line A-A of FIG. 11A. Referring to FIG. 11B, a substrate 210 has a circuit layer 211, an insulating layer 212, and a piezoelectric layer 213 provided thereon in this order, and thin-wall portions 214 and 215 are provided on the back surface of the piezoelectric substrate 210. Referring to FIG. 11A, IDT electrodes 222 and 223 of a first surface acoustic wave element 220 and IDT electrodes 224 and 225 of a second surface acoustic wave element 221 are respectively arranged on portions of the piezoelectric layer 213 corresponding to the thin-wall portions 214 and 215 of the substrate 210.
This surface acoustic wave sensor detects a change in pressure or temperature utilizing a change in the oscillation frequency which is caused by the generation of distortion in the thin-wall portions 214 and 215 of the substrate 210 due to a change in pressure or temperature. By exciting the surface acoustic wave elements 220 and 221 at different frequencies using independent circuits, the influence of a change in the surrounding temperature is compensated for. This enables a high-accuracy pressure and temperature sensor to be obtained. It is stated that examples of the excitation modes of the surface acoustic wave elements 220 and 221 may include a selective combination of a Rayleigh wave mode, a Sezawa wave mode, a second Sezawa wave mode, and a selective combination of high-order excitation modes.
The surface acoustic wave sensor described in WO 2005/003752, which uses SH waves, may not be able to provide sufficient sensitivity for a particle of a detection object and, thus, an increase in the detection sensitivity is desired.
Japanese Unexamined Patent Application Publication No. 2007-57287 does not describe the sensitivity of the sensor. When the configuration described in Japanese Unexamined Patent Application Publication No. 2007-57287 is applied to a liquid-concentration sensor or a biosensor, if a Rayleigh wave or a Sezawa wave having an SV component displacement in a direction vertical to a surface is used for excitation, vibration energy propagates to a sample side when a liquid or a high-viscosity material is loaded on the excited portion. This will cause a large deterioration of characteristics, thereby making the sensor unable to function properly.